The controlled expansion of gases forms the basis for the majority of non-electrical rotational engines in use today. These engines include reciprocating, rotary, and turbine engines, and may be driven by heat (heat engines) or other forms of energy. Heat engines may use combustion, solar, geothermal, nuclear, or other forms of thermal energy. Combustion-based heat engines may utilize either internal or external combustion.
Internal-combustion engines derive power from the combustion of a fuel within the engine itself. Typical internal-combustion engines include reciprocating engines, rotary engines, and turbine engines.
Internal-combustion reciprocating engines convert the expansion of burning gases (typically, an air-fuel mixture) into the linear movement of pistons within cylinders. This linear movement is then converted into rotational movement through connecting rods and a crankshaft. Examples of internal-combustion reciprocating engines are the common automotive gasoline and diesel engines.
Internal-combustion rotary engines use rotors and chambers to more directly convert the expansion of burning gases into rotational movement. An example of an internal-combustion rotary engine is the Wankel engine, which utilizes a triangular rotor that revolves in a chamber, instead of pistons within cylinders. The Wankel engine has fewer moving parts and is generally smaller and lighter, for a given power output, than an equivalent internal-combustion reciprocating engine.
Internal-combustion turbine engines direct the expansion of burning gases against a turbine, which then rotates. An example of an internal-combustion turbine engine is a turboprop aircraft engine, in which the turbine is coupled to a propeller to provide motive power for the aircraft.
Internal-combustion turbine engines are often used as thrust engines, where the expansion of the burning gases exit the engine in a controlled manner to produce thrust. An example of an internal-combustion turbine/thrust engine is the turbofan aircraft engine, in which the rotation of the turbine is typically coupled back to a compressor, which increases the pressure of the air in the air-fuel mixture and markedly increases the resultant thrust.
All internal-combustion engines of this type suffer from poor efficiency. Only a small percentage of the potential energy is released during combustion, i.e., the combustion is invariably incomplete. Of that energy released in combustion, only a small percentage is converted into rotational energy. The rest must be dissipated as heat.
If the fuel used is a typical hydrocarbon or hydrocarbon-based compound (e.g., gasoline, diesel oil, or jet fuel), then the partial combustion characteristic of internal-combustion engines causes the release of a plethora of combustion by-products into the atmosphere in the form of an exhaust. In order to reduce the quantity of pollutants, a support system consisting of a catalytic converter and other apparatuses is often necessitated. Even when minimized, a significant quantity of pollutants is released into the atmosphere as a result of incomplete combustion.
Because internal-combustion engines depend upon the rapid (i.e., explosive) combustion of fuel within the engine itself, the engine must be engineered to withstand a considerable amount of pressure and heat. These are drawbacks that require a more robust and more complex engine over external-combustion engines of similar power output.
External-combustion engines derive power from the combustion of a fuel in a combustion chamber separate from the engine. A Rankine-cycle engine typifies a modern external-combustion engine. In a Rankine-cycle engine, fuel is burned in the combustion chamber and used to heat a liquid at a substantially constant pressure. The liquid is vaporized to become the desired gas. This gas is passed into the engine, where it expands. The desired rotational power is derived from this expansion. Typical external-combustion engines also include reciprocating engines, rotary engines, and turbine engines.
External-combustion reciprocating engines convert the expansion of heated gases into the linear movement of pistons within cylinders. This linear movement is then converted into rotational movement through linkages. The conventional steam locomotive engine is an example of an external-combustion open-loop Rankine-cycle reciprocating engine. Fuel (wood, coal, or oil) is burned in a combustion chamber (the firebox) and used to heat water at a substantially constant pressure. The water is vaporized to become the desired gas (steam). This gas is passed into the cylinders, where it expands to drive the pistons. Linkages (the drive rods) couple the pistons to the wheels to produce rotary power. The expanded gas is then released into the atmosphere in the form of steam. The rotation of the wheels propels the engine down the track.
External-combustion rotary engines use rotors and chambers instead of pistons, cylinders, and linkage to more directly convert the expansion of heated gases into rotational movement.
External-combustion turbine engines direct the expansion of heated gases against a turbine, which then rotates. A modern nuclear power plant is an example of an external-combustion closed-loop Rankine-cycle turbine engine. Nuclear fuel is “burned” in a combustion chamber (the reactor) and used to heat water. The water is vaporized to become the desired gas (steam). This gas is directed against a turbine, which then rotates. The expanded steam is then condensed back into water and made available for reheating. The rotation of the turbine drives a generator to produce electricity.
External-combustion engines may be made much more efficient than corresponding internal-combustion engines. Through the use of a combustion chamber, the fuel may be more thoroughly consumed, releasing a significantly greater percentage of the potential energy. More thorough consumption means fewer combustion by-products and a significant reduction in pollutants.
Because external-combustion engines do not themselves encompass the combustion of fuel, they may be engineered to operate at a lower pressure and a lower temperature than comparable internal-combustion engines. This in turn allows the use of less complex support systems (e.g., cooling and exhaust systems), and results in simpler and lighter engines for a give power output.
Typical turbine engines operate at high rotational speeds. This high rotational speed presents several engineering challenges that typically result in specialized designs and materials. This adds to system complexity and cost. Also, in order to operate at low-to-moderate rotational speeds, turbine engines typically utilize a step-down transmission of some sort. This, too, adds to system complexity and cost.
Similarly, reciprocating engines require linkage to convert linear motion to rotary motion. This results in complex designs with many moving parts. In addition, the linear motion of the pistons and the motions of the linkages produce significant vibration. This vibration results in a loss of efficiency and a decrease in engine life. To compensate, components are typically counterbalanced to reduce vibration. This results in an increase in both design complexity and cost.
Typical heat engines depend upon the diabatic expansion of the gas. That is, as the gas expands, it loses heat. This diabatic expansion represents a loss of energy.
What is needed, therefore, is an external-combustion rotary heat engine that maximizes and utilizes the adiabatic expansive energy of the gases.